1. Technical Field
The present invention relates to methods for measurement, analysis and assessment of ground structure.
2. Background Art
To determine the structure and characteristics of the surface layer of the ground, it has heretofore been necessary to drill a plurality of holes in the ground and insert or indwell instruments and ancillary devices into the holes and also to employ a source of oscillation for generating waves to be measured. It should, therefore, be obviously of great benefit if it were ever possible to know the structure and characteristics of the surface layer of the ground without any of such efforts and contrivances. To this end, it might be contemplated to measure and analyze the oscillatory waves or microseisms observed in the surface layer of the ground. However, up to the present time, there is not known a measuring technology or a system of instrumental analysis, by means of which short-period microseisms could be measured with sufficient accuracy to estimate the tectonic and other characteristics of the surface layer of the ground inclusive of the velocity of shear (S) waves.
Furthermore, the prior art system designed to simultaneously measure short-period microseisms at a plurality of points to thereby delineate the ground structure has the following disadvantages.
A. Since the spacings of oscillation sensors must be large, that is to say the scale of measurement must be large, a large free area is required and it is time-consuming to connect the cords from the respective oscillation sensors for cental analysis. Moreover, because the large number of oscillation sensors are required, the analytical procedure is scaled up to the extent that the method cannot be utilized advantageously in the field. PA0 B. Since the network of oscillation sensors is not tailored to improvement in measuring accuracy or resolution of high-order modes, the reliability of the measured dispersion curve is low. PA0 C. Short-period microseisms include not only substantive waves such as dilatational (P-) waves and S-waves but surface waves such as Rayleigh and Love waves. Since the confirmation or judgement of whether the detected waves are surface waves or not is not performed at measurement or analysis, it is impossible to judge whether the dispersion curve obtained is relevant to the surface waves and, hence, to accurately determine the ground structure. PA0 D. Since short-period waves include higher-modes surface waves, it is impossible to compute an accurate surface wave dispersion curve and, hence, to ascertain the surface layer structure of the ground unless some means for removing or separating these waves are available. PA0 E. Since the dispersion curve cannot be computed real-time from field values, there is no knowing whether measurements were made with acceptable accuracy.